Accept-Language: is always en-us

I notice that the Accept-Language of the browser is always en-us regardless of the location. While the language preference of the browser may not always be set by a user it often is in regional OS builds. I would have expected to see a reference for Chinese from servers in China. A use of en-au from Australia etc.

The language preference of the request is helpfully shown in the test results as part of the header. Would it not be better to be setting this by region?

Have a search for the following string in a set of test results from a non US location to see what I mean.

Accept-Language: en-us